MEF-导入词典

Question

我目前正在重构一个应用程序，想引入 MEF。 Export class (class Apple) 完成并标有 Export-keyword... 在导入站点上，我目前有一个初始化如下所示的字典：

Dictionary<int, Apple> dict = new Dictionary<int, Apple>();
for(int i=0; i < 10; i++)
    dict.add(i, new Apple());

...

如何使用 MEF 初始化字典？

Answer 1

所以我认为你的问题可以归结为，"How to I ensure that the container produces multiple instances of an object rather than reusing the same object every time it's requested." 嗯，这很简单——你只需要指定 CreationPolicy.NonShared。

考虑 IApple 的这个示例实现：

public interface IApple { }

[PartCreationPolicy(CreationPolicy.NonShared)]
[Export(typeof(IApple))]
public class Apple : IApple 
{ 
    private static int appleCounter = 0;
    private int id;

    public Apple() 
    {
        this.id = ++appleCounter;
    }

    public override string ToString()
    {
        return "Apple #" + this.id.ToString();
    }
}

您可以通过以下方式使用它：

class Program
{
    public static void Main(string[] args)
    {
        var catalog = new ApplicationCatalog();
        var container = new CompositionContainer(catalog);
        IDictionary<int, IApple> dict = new Dictionary<int, IApple>();
        for (int i = 0; i < 10; i++)
        {
            dict.Add(i, container.GetExportedValue<IApple>());
        }

        foreach (var pair in dict)
        {
            Console.WriteLine("{0}: {1}", pair.Key, pair.Value);
        }
    }
}

这里的关键代码行是[PartCreationPolicy(CreationPolicy.NonShared)]。没有这个，唯一的 Apple 将被创建。当然，这并不像您希望的那样有用。这是另一种更灵活的生成字典的方法：

public interface IBasket
{
    IDictionary<int, IApple> GetAppleDictionary();
}

[Export(typeof(IBasket))]
public class Basket : IBasket
{
    private IDictionary<int, IApple> dict;

    [ImportingConstructor]
    public Basket([Import] CompositionContainer container)
    {
        this.dict = new Dictionary<int, IApple>();
        for (int i = 0; i < 10; i++)
        {
            this.dict.Add(i, container.GetExportedValue<IApple>());
        }
    }

    public IDictionary<int, IApple> GetAppleDictionary()
    {
        return dict;
    }
}

class Program
{
    [Import(typeof(IBasket))]
    private IBasket basket = null;

    public static void Main(string[] args)
    {
        var program = new Program();
        program.Run();
    }

    private void Run()
    {
        var catalog = new ApplicationCatalog();
        var container = CreateCompositionContainer(catalog);
        container.ComposeParts(this);
        foreach (var pair in this.basket.GetAppleDictionary())
        {
            Console.WriteLine("{0}: {1}", pair.Key, pair.Value);
        }
    }

    private static CompositionContainer CreateCompositionContainer(ComposablePartCatalog catalog)
    {
        var wrappedCatalog = new AggregateCatalog(catalog, new TypeCatalog(typeof (CompositionContainer)));
        var container = new CompositionContainer(wrappedCatalog);
        container.ComposeExportedValue(container);

        return container;
    }
}

这里的棘手部分是 CreateCompositionContainer。此方法确保 CompositionContainer 本身可用于满足对其正在组合的对象的导入。这允许 Basket 直接操作容器来生成它需要的所有苹果。

并且仅出于演示目的，您还可以使用 [ImportMany] 属性来完成类似的事情（尽管所有这些 [Export] 确实让我感到畏缩）：

public interface IApple { }

[PartCreationPolicy(CreationPolicy.NonShared)]
[Export(typeof(IApple))]
[Export(typeof(IApple))]
/* ..repeat N times.. */
[Export(typeof(IApple))]
public class Apple : IApple 
{ 
    private static int appleCounter = 0;
    private int id;

    public Apple() 
    {
        this.id = ++appleCounter;
    }

    public override string ToString()
    {
        return "Apple #" + this.id.ToString();
    }
}

class Program
{
    [ImportMany(typeof(IApple))]
    private IEnumerable<IApple> apples = null;

    public static void Main(string[] args)
    {
        var program = new Program();
        program.Run();
    }

    void Run()
    {
        var catalog = new AssemblyCatalog(this.GetType().Assembly);
        var container = new CompositionContainer(catalog);
        container.ComposeParts(this);
        apples.Dump();
    }
}

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